The first polymer based (resin-based) dental filling materials essentially comprised unfilled polymethylmethacrylate. Although these materials showed certain improvements on the filling materials used up to that time, the polymer was soft and exhibited very high wear and shrinkage. A further improvement in properties of the polymer based filling materials was obtained when an inorganic filler was incorporated into the polymer, and this lead to dramatically reduced wear and shrinkage, and generally more favourable physical properties. This class of dental filling material became known as composites. It is now well known that for a cured composite, the surface hardness increases and the degree of shrinkage decreases as the filler content of the composite is increased. A high surface hardness is desirable because this tends to decrease abrasive wear. Shrinkage of a dental filling material on curing has the consequence that the filling pulls away from the tooth surface on curing, and forms a gap between the filling and the tooth. Bacteria and fluids are able to penetrate between the filling and the tooth, which often leads to renewed decay of the tooth. A low shrinkage for a dental filling material is therefore desirable because this reduces the size of the gap formed between the filling material and the tooth, and therefore reduces the propensity for new decay of the tooth. The upper filler content of a composite is however generally limited because with high filler levels the composite material becomes too stiff. In extreme cases it is difficult or impossible to form a homogenous paste, and even if a paste is obtained it may be too stiff to be used. One problem with dental composite filling materials is therefore to obtain pastes with high filler loading, while still retaining a consistency that allows easy use.
The adduct between hydroxypropylmethacrylate and succinic acid has been used to promote adhesion to tooth substance (Dep. Dent. Mater. Dev., Fukuoka Dent. Coll., Fukuoka, 814-01) and was mentioned in patents JP3021603 and JP53051237 as an adhesion promoter to teeth. In WO 97/29732 adducts of 2-hydroxyethylmethacrylate (HEMA) with various anhydrides are mentioned, in particular the adduct of hydroxyethylmethacrylate with glutaric and succinic anhydrides, for dental cements or ionomers. The former adduct is termed GMA in WO 97/29732. However, WO 97/29732 describes water-containing dental cements containing adducts of HEMA with various aryl anhydrides that contain at most 76.85% filler by weight (about 55% filler by volume using an overall density of the metallic filler powder of about 5.5 g/cm3), a level which is easily reached using conventional resin mixtures containing no such adducts. No mention is made of a possibility of obtaining high volume-% filler loadings. WO 97/29732 is not concerned with dental composites nor with the problem of increasing the filler load of dental composites.
The use of dispersing agents to aid incorporation of a filler into a liquid matrix is known. For instance, in U.S. Pat. No. 4,407,984 and EP0053442 it is mentioned that a dispersing agent is desirable to assist in dispersing a filler into a composite material. In U.S. Pat. No. 4,407,984 and EP0053442 dispersing agents based on phosphoric acid esters and alkyl amines are mentioned. U.S. Pat. No. 6,300,390 mentions the use of a phosphate ester dispersing agent. As particularly preferred dispersants are mentioned phosphoric esters containing polymerizable groups, or phosphate esters containing a carboxylic ester group and an ether group. As demonstrated in the examples, such phosphate esters have a very low pH which is deleterious to some dental glasses. The filler load achieved in U.S. Pat. No. 6,300,390 is, however, insufficient and does not provide for high mechanical strength after polymerization. If the filler load in the compositions of U.S. Pat. No. 6,300,390 is increased, the viscosity becomes too high and no homogenous paste can be formed.
It is therefore a problem of the invention to provide a polymerizable dental composite having an increased filler content, while still having a sufficiently low viscosity for easy use. It is another problem of the invention to provide a polymerizable dental composite exhibiting reduced shrinkage upon polymerization while having a good or improved strength after polymerization. It is another problem to provide a polymerizable dental composite having an increased filler load and, good handling properties and giving, upon polymerization, a polymerized dental composite having good physical properties.